The present invention relates to a process for producing titanium oxide. Specifically, the present invention relates to a process for producing titanium oxide suitable for use for a photocatalyst.
It has been investigated that malodorous substances in air are removed and that window glass and driveway walls are cleaned by a photocatalytic activity shown by titanium oxide. Recently, a decomposition and removal method using visible light as a light source has been drawing attention owing to the wide usability and the convenience and, therefore, titanium oxide exhibiting a high photocatalytic activity by irradiation of visible light has been developed.
Although a variety of processes as a method for producing such a titanium oxide have been proposed, there are problems for the production such that a specific apparatus equipped with a vacuum container is required and that the obtained titanium oxide is insufficient in the photocatalytic activity. The above-described problems especially become obstacles when a titanium oxide having a small particle size is needed, for example, when the titanium oxide is used by being uniformly applied to automotive materials or construction materials such as window glass and driveway walls.
One of the purposes in the present invention is to provide a process for easily producing a titanium oxide showing a high photocatalytic activity by visible light radiation without using a specific apparatus equipped with a vacuum container. Preferably, the purpose of the present invention is to provide a process for easily producing a titanium oxide having a small particle size.
Inventors of the present invention have investigated the process for producing titanium oxide and completed the present invention.
That is, the present invention provides a process for producing a titanium oxide which comprises the steps of (i) reacting a titanium compound with ammonia in an amount exceeding the stoichiometric amount necessary to convert the titanium compound to a titanium hydroxide or reacting a titanium compound with a base at pH of from about 2 to about 7, to obtain a reaction product and (ii) calcining the obtained product.
A titanium oxide in the present invention is produced by a process which comprises the steps of (i) reacting a titanium compound with ammonia in an amount exceeding the stoichiometric amount necessary to convert the titanium compound to a titanium hydroxide or reacting a titanium compound with a base at pH of from about 2 to about 7, to obtain a reaction product and (ii) calcining the obtained product.
A titanium compound to be used for the present invention may be any titanium compound capable of producing a titanium hydroxide by reaction with a base and examples thereof are titanium trichloride (TiCl3), titanium tetrachloride (TiCl4), titanium sulfate [Ti(SO4)2 mH2O, 0xe2x89xa6mxe2x89xa620], titanium oxysulfate [TiOSO4.nH2O, 0xe2x89xa6n less than 20], titanium oxychloride (TiOCl2) and the like. Among them, titanium oxysulfate is preferably used. A titanium compound having a high purity is preferably used and a titanium compound having a purity of 99% by weight or higher is more preferably used. By using a titanium compound with a high purity, a minute particulate titanium oxide exhibiting a high photocatalytic activity may be obtained. The purity of the titanium compound may be measured by JIS K8401-1992 method in the case of titanium trichloride and by JIS K8460-1992 method in the case of titanium tetrachloride. In the case of titanium oxysulfate, the purity thereof may be calculated by measuring the contents of TiO2 and SO3 as main components and the contents of impurities such as SiO2, P2O5 and Nb2O5 to obtain the purity in accordance with the following equation:
Purity(%)=[(A1+A2)/(A1+A2+A3+A4+A5+ . . . )]xc3x97100,
wherein A1 and A2 respectively represent the contents (% by weight) of TiO2 and SO3 and A3, A4, A5, . . . respectively represent the contents (% by weight) of impurities such as SiO2, P2O5 and Nb2O5.
One (the first production process) of the production processes in the present invention comprises a step of reacting a titanium compound with ammonia in an amount exceeding the stoichiometric amount necessary to convert the titanium compound to a titanium hydroxide prior to the calcining step.
In the case that the titanium compound is reacted with ammonia in an amount exceeding the stoichiometric amount necessary to convert the titanium compound to a titanium hydroxide, the stoichiometric amount is practically equivalent to the amount (by mole) of (Bxc3x97C) times as much as the molar amount of titanium compound to be converted, the (Bxc3x97C) being calculated by multiplying the number B of the acid radicals contained in one molecule of the titanium compound by the valence C of the acid radicals. The stoichiometric amount necessary to convert the titanium compound to a titanium hydroxide is the amount of, for example, 3 times (by mole) in the case of using titanium trichloride, 4 times (by mole) in the case of using titanium tetrachloride, 4 times (by mole) in the case of using titanium sulfate, 2 times (by mole) in the case of using titanium oxysulfate, and 2 times (by mole) in the case of titanium oxychloride as much as the molar amount of titanium compound to be converted. In the first production process, a titanium compound is reacted with ammonia in an amount exceeding the stoichiometric amount and the amount of ammonia may be that of about 1.2 times or more, preferably about 2 times or more as much as the stoichiometric amount, and abut 20 times or less, preferably about 10 times or less as much as the stoichiometric amount.
The reaction of a titanium compound with ammonia may be carried out at about 70xc2x0 C. or lower. The temperature is preferably about 40xc2x0 C. or lower and more preferably about xe2x88x925xc2x0 C. or lower. The reaction may be carried out in a process comprising the steps of placing a titanium compound in a reaction container, adding ammonia, e.g. ammonia solution (ammonia water) while stirring them and mixing them, in a process comprising the steps of placing ammonia in a reaction container, adding a titanium compound while stirring them and mixing them, or in a process comprising the steps of simultaneously placing a titanium compound and ammonia in a reaction container and mixing them.
Another one (the second production process) of the production processes in the present invention comprises a step of reacting a titanium compound with a base at pH of from about 2 to about 7 prior to the calcining step. In this process, the pH is the pH value of the mixed solution or slurry of the titanium compound and the base. The reaction may be carried out thoroughly (from the starting to the finishing) at pH of from about 2 to about 7.
For this reaction, any process of converting a titanium compound to a titanium hydroxide may be employed. The rection may be conducted in a process comprising the steps of placing an aqueous medium in a reaction container and then supplying an aqueous solution of a titanium compound and a base to the reaction container while stirring them or in a process comprising a step of continuously supplying an aqueous medium, a titanium compound and a base to a reaction tube. The yield of titanium hydroxide may be about 90% or higher and preferably about 95% or higher. The reaction may be carried out at pH of about 2 or higher and at pH of about 7 or lower. When the pH of the reaction is higher than about 7, the particle size of the resulting titanium oxide, which is obtained by calcining the reaction product of titanium compound with base, tends to be large. The pH of the reaction is preferably about 2.5 or higher, more preferably about 3 or higher and is preferably about 5.5 or lower, more preferably about 5 or lower.
Examples of the base to be reacted with the titanium compound at pH of from about 2 to about 7 include aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate or ammonia and besides them, the examples also include aqueous solutions of hydrazine, hydroxylamine, monoethanolamine, an acyclic amine compound, a cyclic aliphatic amine compound or the like. Among them, the aqueous solution of ammonia (ammonia solution) is preferably used. When a base is used in the form of an aqueous solution, the concentration of the base in the aqueous solution may be about 0.05% by weight or higher and about 50% by weight or lower.
The reaction of a titanium compound with a base at pH of from about 2 to about 7 may be carried out at about 5xc2x0 C. or higher, preferably at about 20xc2x0 C. or higher and may be carried out at about 90xc2x0 C. or lower, preferably at about 70xc2x0 C. or lower. When the reaction temperature is higher than about 90xc2x0 C., the reaction between the titanium compound and the base may be locally proceeded and the reaction product may be unevenly produced so that the particle size of the finally obtained titanium oxide tends to become large.
The product obtained by the reaction at pH of from about 2 to about 7 is preferably subjected to aging thereafter by being further mixed with a base. The aging may be carried out in a process comprising a step of keeping the slurry containing the product in the presence of a base in a constant temperature range. For the aging, a base may be specially added to the slurry containing the product, the slurry being prepared by separating the product from a solution containing the resulting salt (e.g. ammonium sulfate or the like) produced by the reaction and then dispersing the product. By carrying out such aging for the product, the titanium oxide obtained after calcining the product may have a shape of minute particles and show an excellent photocatalytic activity under visible light radiation.
A base to be used for the reaction at pH of from about 2 to about 7 and a base to be used for the aging may or may not be the same kind of ones, and the contents of them in the solution (in the case of being added in a solution form) may or may not be the same. An example of a preferable base to be employed for aging is ammonia solution and the ammonia content of the ammonia solution may be about 0.05% by weight or higher and about 50% by weight or lower. The aging temperature may be about 0xc2x0 C. or higher, preferably about 10xc2x0 C. or higher, and may be about 110xc2x0 C. or lower, preferably about 80xc2x0 C. or lower. The period of time for aging is not limited and varies depending on a base content and an aging temperature. The period of time may be about 0.01 hour or longer, preferably about 0.5 hour or longer and may be within 60 hours, preferably within about 24 hours.
The total amount of a base to be used for the reaction at pH of from about 2 to about 7 and a base to be used for aging is preferably an amount exceeding the stoichiometric amount necessary to convert a titanium compound to a titanium hydroxide. Specifically, the total amount may be that of about 1.1 times by mole or more, preferably about 1.5 times by mole or more, and about 20 times by mole or less, preferably about 10 times by mole or less, as much as the stoichiometric amount. The larger the total amount of bases to be used is, the higher the photocatalytic activity of the finally obtained minute particulate titanium oxide tends to be. Nonetheless, when the amount exceeds a certain amount that extremely exceeds the stoichiometric amount, it may become difficult to obtain the photocatalytic activity corresponding to the amount.
The product obtained by reacting a titanium compound with ammonia in an amount exceeding the stoichiometric amount, the product obtained by reacting a titanium compound with a base at pH of from about 2 to about 7, or the product obtained by aging each of the products may be obtained in a slurry of a reaction mixture. The slurry containing the product may be used for the calcining step as it is or after the slurry is subjected to solid-liquid separation and the separated solid is washed if necessary. The separation may be carried out by pressure filtration, vacuum filtration, centrifugal separation, decantation or the like. Alternatively, the separation may be carried out by heating the slurry with a pneumatic conveying drying to evaporate the liquid therein.
The obtained slurry or the product recovered from the slurry by separation operation is then calcined. The calcination may be carried out at about 300xc2x0 C. or higher, preferably at about 350xc2x0 C. or higher, and may be carried out at about 600xc2x0 C. or lower, preferably at about 500xc2x0 C. or lower. When the calcining temperature is too high, the photocatalytic activity of the resulting titanium oxide may decrease. The calcination may be carried out in a pneumatic conveying furnace, a tunnel furnace, a rotary furnace or the like.
The titanium oxide obtained by the process of the present invention may have an anatase-type crystal structure. The titanium oxide exhibits a photocatalytic activity by radiation of ultraviolet light and/or visible light. The titanium oxide exhibiting an especially high photocatalytic activity by visible light radiation may be a titanium oxide which is obtained by a process comprising the steps of (i) reacting a titanium compound with ammonia in an amount exceeding the stoichiometric amount or reacting a titanium compound with a base at pH of from about 2 to about 7, to obtain a reaction product, (i)xe2x80x2 adding an additional base to the product to conduct aging and (ii) calcining the product. The obtained titanium oxide may be used for a variety of applications such as for decomposing NOx in atmospheric air, for decomposing and removing malodorous substances and mold or for decomposing and removing an organic solvent in water, as it is or after a molding processing. In addition, the titanium oxide obtained after calcining the reaction product by reacting a titanium compound with a base at pH of about 2 to about 7 may be minute particles having an average particle size of about 20 xcexcm or smaller and may be suitably utilized as a raw material for a coating material that is uniformly applied onto a substrate.
As described above, titanium oxide having a high photocatalytic activity can be easily obtained by the production process of the present invention. A minute particulate titanium oxide with the average particle size of about 20 xcexcm or smaller and showing a photocatalytic activity can be also easily obtained. Further, by combining the aging step, a minute particulate titanium oxide having a more excellent in photocatalytic activity by the radiation of the visible light can easily be obtained.
The process for producing titanium oxide of the present invention is described in Japanese application nos. 2000-230779, filed on Jul. 31, 2000 and 2000-388333, filed on Dec. 21, 2000, the complete disclosures of which are incorporated herein by reference.